1. Field of the Invention
The present invention relates in general to a semiconductor process, more particularly, to a method for forming a bottle-shaped trench and a method for forming a bottle-shaped trench capacitor.
2. Description of the Related Art
Typically, the capacitors most widely used in dynamic random access memory (DRAM) comprise two conductive layers (electrode plates) having an insulating layer in between. The ability to store the electric charge of a capacitor depends on the thickness of the insulating layer, surface area of the electrode plate and the electrical characteristics of the insulation material.
Due to recent demand for reduced size of semiconductor elements, for enhancing integration of integrated circuits, the area of cells in a memory device must continuously be reduced to support a larger number of memory cells, thereby increasing integration. Meanwhile, the electrode plates of a capacitor in a memory cell must have a sufficiently large surface area to store adequate electrical charge.
Nevertheless, as the size of elements is continuously reduced, trench storage node capacitance of DRAM has also decreased. As a result, storage capacitance must be increased to maintain good operating performance in memory devices. Currently, the method for increasing storage capacitance for DRAMs increases the width of the bottom of the trench, thereby increasing surface area to form a bottle-shaped trench capacitor.
The above method is carried out by selective oxidation of the upper portion of a trench to form a collar oxide layer to protect the upper portion of the trench. Next, the lower portion of the trench is wet-etched to form a bottle-shaped trench having a greater diameter than the upper portion of the trench.
In a conventional process, a trench is formed by isotropic dry etching on a semiconductor substrate having a masking layer comprised of an oxide layer and an overlying nitride layer formed thereon. Next, a nitride layer, an oxide layer, polysilicon layer and another oxide layer are sequentially formed on the masking layer and the trench. Nevertheless, the multiple deposition steps further increase the complexity of the process, thus incurring high production costs and lengthening process time. Hence, a simplified process with high production yield for fabricating bottle-shaped trench capacitors is required. In addition, in order to accomplish next generation, high performance memory devices, a method for increasing capacitance of the bottle-shaped trench capacitors is also required.